Sulphate attack occurs when sulphates react with hardened cement paste, causing expansion and cracking of concrete. Soil sulphates do not severely damage concrete, but water sulphates can enter porous concrete and react with hydration products. This forms ettringite which increases in volume, disintegrating the concrete. Sulphate attack can be external from sulphates in groundwater penetrating concrete, or internal from sulphates in the original mix. Delayed ettringite formation is a type of internal sulphate attack where ettringite decomposes during curing then reforms, expanding and damaging the concrete.

1. SULPHATE
ATTACK

2. Introduction
• Sulphates occur in both soil and ground water.
• Soild sulphate does not attack the concrete severely.
• But water sulphates enter into the porous concrete and
react with the HCP products forming a whitish
appearance.
• This indicates Sulphate Attack.

3. Sulphate Attack
• Increase in the volume of cement paste in concrete or
mortar due to the chemical reaction between the
products of HCP and solution containing sulphates.
• In hardened concrete, C-A-H can react with sulphate
salt from outside, forming calcium sulphoaluminate in the
framework of HCP.
• Due to the increase in volume of the solid phase( which
can go up to 227%),a gradual disintegration of concrete
take place.

4. • Sulphate attack manifest in the form of expansion &
cracking of concrete.

5. Reactions
Reactions on hardened cement paste
• Sodium sulphate attacking Ca(OH)₂
Ca(OH) +Na SO .10 H O CaSO .2 H O +2NaOH +8H O₂ ₂ ₄ ₂ ₄ ₂ ₂
• Reaction with Calcium aluminate hydrate
2(3CaO . Al O . 12 H O) + 3(Na₂ ₃ ₂ ₂SO₄. 10H₂O)
3CaO. Al₂O₃. 3CaSO₄. 31H₂O + 2Al(OH)₃ +
( ettringite) 6NaOH +
17H₂O

6. • Calcium sulphate reacts with CAH to form calcium
sulphoaluminate( ettringite).
• Magnesium sulphate reacts with Ca(OH) , CAH and₂
completely decomposes CSH making it a friable mass
(easy to crumble).

7. Types
» External
» Internal
External Sulphate Attack
• Due to the penetration of sulphates
from a solution
(groundwater) into the concrete
from outside.
• Composition and microstructure of
concrete changes.

8. • These changes may vary in type or severity but
commonly include:
• Extensive cracking
• Expansion
• Loss of bond between the cement paste and aggregate
• This results in overall decrease in strength.

9. Other sources of sulphate which can cause sulphate attack
include:
• Seawater
• Oxidation of sulphate minerals in clay adjacent to the concrete -
this can produce sulphuric acid which reacts with the concrete
• Bacterial action in sewers - anaerobic bacterial produce sulphur
dioxide which dissolves in water and then oxidizes to form
sulfuric acid
• In masonry, sulphates present in bricks and can be gradually
released over a long period of time, causing sulphate attack of
mortar, especially where sulphates are concentrated due to
moisture movement

10. Scanning electron microscope image of sulphate attack in concrete.

11. • Ettringite (arrowed) has replaced some of the calcium
silicate hydrate in the cement paste.
• The darker areas of paste have been partly decalcified.
• As a consequence of these alterations, the paste will be
weakened.

12. Internal Sulphate Attack
• Due to source of sulphate being incorporated into the
concrete at the time of mixing, while adding gypsum in
the cement etc
• Proper screening and testing procedures should
generally avoid internal sulfate attack.

13. Delayed ettringite formation
• Delayed ettringite formation (DEF) is a special case of
internal sulfate attack.
• DEF occurs if the ettringite which normally forms during
hydration is decomposed, then subsequently re-forms in the
hardened concrete.
• Sulphate ions released by decomposition of ettringnite are
absorbed by CSH. Then sulphate ions are desorbed,
reformation of ettringnite take place.
• Damage to the concrete occurs when the ettringite crystals
exert an expansive force within the concrete as they grow.

14. • DEF causes a characteristic form of damage to the concrete.
While the paste expands, the aggregate does not.
Delayed ettringite formation: scanning electron microscope image of
limestone aggregate particle

15. • The cement paste has expanded and a gap has formed
between between the aggregate and the cement paste.
• The aggregate is no longer contributing to concrete strength,
since it is effectively detached from the cement paste.
• Often, these gaps become filled with ettringite.

16. Conditions necessary for DEF to occur are:
• High temperature (above 65-70 degrees C approx.), usually
during curing but not necessarily
• Water: intermittent or permanent saturation after curing
• Commonly associated with alkali-silica reaction (ASR)
• Limestone coarse aggregate has been found to reduce
expansion.

17. Reference
• Concrete Technology by M S Shetty.
• Concrete, Microstructure, Properties and Materials by
Metha, P K and Monteiro.
• Understanding Cement by Nicholas B Winter.